Silicon and Silicon based alloys are well known and are used in a variety of applications. Currently, these alloys are created by melting a predetermined amount of silicon and a combination of metals together into a molten metal liquid. The molten metal liquid is then disposed in a cast or cast iron chill and cooled to form a resultant ingot, which is then crushed via a mechanical means to produce a silicon or silicon-based alloy product. The product is then screened and sized to obtain a desired screen distribution.
Unfortunately, however, these current production methods have a number of problems and produce a product having a number of undesirable characteristics. For example, because these methods include slowly cooling the cast containing the molten metal liquid, the individual grains of the inoculants are typically full of porosity and gas holes. Moreover, the microstructure also tends to reveal that the different metallurgical phases into which the alloying elements are distributed are not very well distributed, resulting in a coarse segregation of the alloying elements. Additionally, the planes of material fracture appear to be richer in impurities and the shape of the product is irregular and often consists of splintered pieces. Lastly, an appreciable amount of dust is created during the crushing and sizing process.
One method that may be used to increase the quality of the product relates to the use of atomization. Atomization is a well-known process that involves introducing a pressurized volume of gas and/or liquid with a liquid molten stream to cause the formation of droplets that are usually smaller than 100-mesh in size. Currently, conventional water and gas atomization processes are the most popular methods and presently account for the bulk of existing atomized materials, where materials produced using water atomization have a high surface oxygen content and materials produced using inert gas atomization have a low surface oxygen content. The high surface oxygen content resulting from the water atomization process is mainly due to the oxygen content in water. This is why the gas atomization process, which uses a gas without oxygen or with a very low oxygen content, is preferred. A low surface oxygen content advantageously allows for the element to be better dispersed throughout the product which results in a more efficient grain distribution structure and thus an improved reactivity.
Typically, the major components of an atomization facility include a melter, an atomization chamber and a dryer (for water atomization). The melter is used to melt a metal or combination of metals into a molten metal liquid that is poured into a tundish, which acts as a reservoir. The metal may be melted following standard protocols where induction melting, smelting or fuel-heating are suitable procedures. The molten metal is then funneled from the tundish, through a metered nozzle having a size that is responsive to the freezing characteristics of the molten metal and into the atomization chamber where a high or low-pressure atomizing nozzle impinges upon the molten metal stream. This causes the formation of molten metal droplets having various sizes, wherein the droplet size is dependent upon the pressure and volume of the impinging stream of gas and/or liquid. Thus, by regulating the parameters of the atomization process, such as the pressure and molten flow rates, the particle size, the particle size distribution, the particle shape, the chemical composition and the microstructure of the particles may be controlled and varied as desired.
The molten metal stream is then cooled by falling vertically through the atomization chamber, wherein the atomization chamber is filled with an inert gas, an inert liquid and/or a combination thereof. The cooled droplets of molten metal may then be sent to a wet screening system or to a drying system, where if the drying system is a gas drying system, horizontal cooling tubes may also be used to cool the molten metal droplets.
However, although the atomization process has existed since approximately 1945, applying the process with different alloys while regulating the atomization process parameters to control the resulting composition (such as producing 40% to 80% by weight silicon alloys) has not been tried until recently and may be particularly advantageous when trying to produce a product whose content requires a controlled distribution of an element, or elements, by weight. The advantage of using the atomization process to produce silicon based alloys may be seen by recognizing that conventional water atomization processes typically have high pressure ranges from 600 to 5000 psi depending on the size of particles desired. Whereas, a much lower-pressure atomizing stream may be used to make silicon or silicon based alloys in the desired size range of 5-mesh to 500-mesh.
Thus, there is a need for a method for producing silicon and silicon based alloys in atomized form.